Hydraulic backwashing

Although generally associated with manual cleaning, it is possible to obtain fully automatic bag filter systems. These have a hydraulic backwashing and bag-cleaning device solidly attached to the steel bag housing. 

Silica is usually removed by treatment of the resin with sodium hydroxide solution of about 5 per cent concentration. This cannot easily be accomplished in the column itself and is best performed in a separate vessel. For this purpose, the resin is removed from the column by hydraulic backwashing with water. ... 

Over long periods of operation, membrane fouling is generally not totally reversible by the hydraulic backwash procedure. As the number of filtration cycles increases, the irreversible fraction of membrane fouling also increases (see Fig. 6.13). In order to obtain the desired production flow rates (or flux), an increase in TMP is required. When this pressure reaches a maximum allowable mechanical resistance of the membrane, chemical cleaning is required for the membrane to regain most of its permeability. Regardless of the membrane system... 

During backwash the larger, denser panicles will accumulate at the base and the particle size will decrease moving up the column. This distribution yields a good hydraulic flow pattern and resistance to fouling by suspended solids. 

Hydraulic classification The rearrangement of resin particles in an ion-ex- change unit. As the backwash water flows up through the resin bed, the particles are placed in a mobile condition wherein the larger particles settle and the smaller particles rise to the top of the bed. 

System component Filter beds, media, backwash mechanism, pumps, and controls. The filter beds were sized for hydraulic loading of 81 L/min/m2 (2gpm/ft2). 

In a typical fixed-bed carbon column, the column is similar to a pressure filter and has an inlet distributor, an underdrain system, and a surface wash. During the adsorption cycle, the influent flow enters through the inlet distributor at the top of the column, and the groundwater flows downward through the bed and exits through the underdrain system. The unit hydraulic flow rate is usually 2 to 5 gpm/ft2. When the head loss becomes excessive due to the accumulated suspended solids, the column is taken off-line and backwashed. 

Fig. 1 describes a typical rapid sand filter. Note that hydraulic grading of the sand occurs in the backwash cycle, with the very finest sand forming the top of the bed. Filtration occurs in the top few inches. Once a suspended solid passes the top layer, larger and larger voids occur, and the chance of passing through the filter is greatly increased. 

Backwashing is necessary to keep the bed in a hydraulically classified condition, to minimize pressure drop, and to remove resin fines and suspended solids that have been filtered out of the influent water. Normal practice is to backwash at the end of each run for about 15 min, so as to obtain about 50 to 75 percent bed expansion. The flow rate required to achieve this expansion is obtained from the manufacturers data. As noted in the statement of the example, an appropriate flow rate in this case is 6.4gal/(min)(ft2). The total backwash rate is thus [6.4gal/(min)(ft2)](14ft2) = 90 gal/min. The total water requirement, then, is (90 gal/min)(15 min) = 1350 gal (5.11 m3). 

The resin beads used in most columnar operations range in size from 0.3 to 0.9 mm in diameter, which is a compromise based on the effect of ion-exchange rates, capacities, and hydraulic characteristics. The especially made resins used in resin-in-pulp operations range in size from 0.8 to 1.6 mm in diameter. The apparent density of a resin is defined as that weight of backwashed and settled wet resin per cubic foot, which for resins used in the uranium industry is about 38-45 Ib/ft . In column operations, the attrition losses due to swelling and contraction of resin, abrasion of resin-resin surfaces, and abrasion of resin-equipment surfaces are negligible. In resin-in-pulp operations, an appreciable amount of attrition loss is encountered. 

The treatment or service cycle occurs across a mixed bed contained in the operator vessel. Upon termination of the service cycle all the resin is hydraulically transferred to a separation-regenerator vessel for separation by backwashing. Instead of adopting insitu regeneration in this vessel the anion resin is further physically transferred to another column for separate regeneration, whilst the cation resin is separately regenerated in the separation vessel. Finally, the cation resin is transferred to the anion vessel, air mixed, rinsed, and returned to the operator. 

Microscreens. In the course of this research, we came to the opinion that a preliminary screening to remove most of the biomass was desirable. We shall report evaluations of microscreens, or mlcrostralners, for this purpose. These devices have been available for several decades for waste water treatment (11). They are low-hydraulic-head filters, comprised of a screen mounted on a rotating drum. Apertures of available screens range down to micron sizes. Feed is introduced into the drum. Filter-cake control is by backwash with air or a portion of the filtrate, once each rotation, the backwash being caught in a tray. A schematic is shown in Figure 6. 

Hydraulic Cleaning/ Backwashing Feed Chlorination Feed Acidification Chemical Cleaning... 

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